In modern motor vehicles, electrical loads such as, for example, the interior lighting or window lifter drives are controlled by a microprocessor as a function of the operated states of a plurality of switching elements such as, for example, door contact switches, boot lock switches, ignition lock switches, etc. The processor must therefore first detect the operated states of a plurality of switching elements and initiate the required action accordingly.
To avoid the microprocessor or associated microcontroller circuit arrangement continuously drawing a relatively high power, the microcontroller circuit or microprocessor is set to a standby or sleep mode during the periods in which the processor needs to perform no actions. From this mode, in which the circuit arrangement draws only a very low power, the microprocessor is switched, as required, i.e. on demand, or at predetermined time intervals, to the active mode by means of a wake-up signal.
With known circuit arrangements for detecting the state of at least one electrical actuating element, the relevant operated states are monitored by applying clocked voltage pulses via series resistors in each case to the individual switching elements, with a voltmeter being used to detect a voltage drop across the individual switching elements, said voltage drop reflecting the operated state of the respective switching element. For example, the electrical potential at the input of the voltmeter is connected to ground during switching of a switching element, whereas the electrical potential at the input of the voltmeter corresponds to the open switching elements of the supply voltage.
A disadvantage with this known circuit arrangement for detecting the state of at least one electrical switching element is the fact that an output signal reflecting the state of the switching elements is generated at the signal output even if none of the switching element is actuated and therefore also no output signal would be necessary. This known circuit arrangement is thus always in an active operating state during the clocked voltage pulses, which means that electrical energy is consumed unnecessarily.
A circuit arrangement for detecting operated states of switches for activating a motor vehicle electronics system is known from DE-A-199 17 819, wherein the circuit arrangement generates a wake-up signal if at least one switch is moved from a first defined operated state to a second defined operated state. The circuit arrangement is designed so that it generates the wake-up signal even during a transition from the second operated state to the first operated state. This is frequently necessary in automotive engineering because electrical switches in the active state are often closed. It is therefore necessary for both operated states to be clearly detectable.